1. Field of the Invention
Generally speaking, the invention relates to light-emitting diode (LED)-based linear lighting, and more particularly, to linear lighting with distributed onboard power conversion.
2. Description of Related Art
Over the last decade, lighting based on light-emitting diodes (LEDs) has become dominant in the lighting industry, and is widely used in both residential and commercial installations. LED-based lighting has a number of advantages compared with legacy incandescent and fluorescent lighting, including high efficiency and low power draw, relatively low operating temperatures, and, with some models, selectable color and a wide variety of available color temperatures.
For most commercial and residential applications, two major types of LED-based lighting are used: bulb-type lamps and linear lighting. Bulb-type lamps are intended as direct replacements for incandescent light bulbs, typically have a shape similar to the type of bulb they are intended to replace, and are usually constructed to produce roughly the same light output as the bulbs they are intended to replace. Linear lighting is somewhat different—it usually includes a number of LEDs arranged at a regular spacing or pitch along a printed circuit board (PCB). That PCB may be rigid, made, for example, of FR4 composite, or it may be flexible, made, for example, of Mylar. In either case, the PCB usually has the form of a thin strip, although other shapes and sizes are possible.
One of the major advantages of linear lighting is its versatility. Alone, it can serve as accent lighting or task lighting, often in locations where it would be difficult to install traditional lighting fixtures. Placed in an appropriate extrusion and covered with a diffuser, it can serve as primary room lighting, replacing legacy fluorescent fixtures in offices. Properly electrically insulated and encapsulated, it can be used even in outdoor and wet locations.
In linear lighting, the individual LEDs themselves are usually packaged along with color-modifying elements, such as phosphors, and the packages may also include lensing and light-diffusing elements. LEDs used in linear lighting typically accept direct current (DC) voltages, often at low voltage levels. While the definitions of “low voltage” and “high voltage” depend on the authority one consults, for purposes of this description, voltages over about 50V will be considered to be high voltage.
Many types of linear lighting operate using low voltage DC, for example, 12-24 VDC. For example, the linear lighting disclosed in U.S. Pat. No. 9,239,136, the contents of which are incorporated by reference in their entirety, is intended to operate with low voltage DC. However, the linear lighting has an inherent voltage drop per unit length, which means that the voltage drops progressively lower as one moves from the end of the PCB where power is connected toward the far end of the PCB. Ultimately, this limits the maximum possible length of low-voltage DC linear lighting, because as the PCB gets longer, the voltage at the far end ultimately drops to a level below that needed to activate (i.e., saturate) the LEDs.
One ready solution to the problem of voltage drop in linear lighting is to increase the operating voltage. Although high-voltage AC linear lighting allows for longer runs, it comes with its own challenges. Typically, the LEDs themselves run on low-voltage DC, which means that somewhere and somehow, power must be converted from high-voltage AC to low-voltage DC to power the LEDs. The usual solution is to provide an external driver that transforms and rectifies the voltage. Drivers are often large, and are one more thing that a contractor or electrician must find room for when installing the lighting. Sometimes, placing a driver requires making a large and inconvenient hole in a wall. Better apparatus and methods for providing power for linear lighting would be useful.
Yet merely converting from AC to DC power, while necessary, may not be sufficient to provide good lighting. AC power, by definition, varies cyclically over time. Cyclical variation in power over time is not a problem for incandescent lighting, because the frequency of the AC cycle, typically 60 Hz in the United States and 50 Hz in some other countries, is too rapid to cause much perceptible variation of the light output in incandescent lighting. However, LED light engines can respond much more rapidly to changes in the incoming power. If, for example, the voltage input to an LED is cyclical or ripples over time, this can cause the LED to vary in luminance over time, or to turn on and off rapidly. Depending on the frequency involved, that can create a perceptible flicker. Commercial- and consumer-grade AC-to-DC drivers leave at least some cyclical ripple in their DC output, meaning that most commercial LED fixtures are susceptible to flicker.
Authorities have noted the potentially deleterious effects of flicker in lighting. For example, IEEE Standard 1789-2015, “IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers,” the contents of which are incorporated by reference in their entirety, describes flicker, methods of measuring it, and recommendations for mitigating it.